fix(engine): 全拼合法键入被别名改写后 preedit 不再照搬改写串

engine/core/input_session_composition.cpp

@@ -285,7 +285,13 @@ std::string BuildQuanpinAutocorrectDisplay(const QueryRequest &request)
     // bit rides along with the legacy switches.
     if (quanpin::looks_like_syllable_with_jianpin_tail(request.raw_input))
     {
-        return base;
+        // The typed spelling is legal, but the scheme alias table may still
+        // have re-segmented it into a reading with copied letters (dongua ->
+        // dong'gua, haoyonga -> hao'yong'ga). The dictionary ranks those
+        // readings against the exact one by frequency, yet the preedit must
+        // show the typed letters (PRD R5), so a rewritten base may never
+        // reach it verbatim.
+        return letters_rewritten ? QuanpinLettersWithoutDelimiters(cased) : base;
     }
 
     const auto cut = quanpin::autocorrect_cut_detail(folded_input, types);

engine/tests/src/test_input_session.cpp

@@ -824,6 +824,25 @@ int run_test()
                     "Selecting a corrected phrase left an unconsumed input suffix.");
         }
 
+        // PRD R5 regression: a fully legal spelling whose alias reading won the
+        // query (haoyonga -> hao'yong'ga) must still preedit the typed letters.
+        // The rewritten segmentation used to leak into the preedit verbatim,
+        // adding a phantom letter and shifting the TSF caret (haoyongg|a).
+        struct LegalSpellingCase
+        {
+            const char *pinyin;
+            const char *expected_preedit;
+        };
+        const std::array<LegalSpellingCase, 4> legal_spelling_preedit_cases = {
+            {{"haoyonga", "haoyonga"}, {"dongua", "dongua"}, {"zhonguo", "zhonguo"}, {"dongan", "dongan"}}};
+        for (const auto &test_case : legal_spelling_preedit_cases)
+        {
+            metasequoia::InputSession session;
+            type(session, test_case.pinyin);
+            require(session.get_pinyin_segmentation_with_cases() == test_case.expected_preedit,
+                    "A legal spelling with an alias reading must preedit the typed letters.");
+        }
+
         metasequoia::InputSession no_autocorrect_session(SchemeType::Quanpin, 0u);
         require(no_autocorrect_session.quanpin_autocorrect_types() == 0,
                 "The requested pinyin autocorrect setting was not retained.");